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PTYS 554 Evolution of Planetary Surfaces Volcanism I

PTYS 554 Evolution of Planetary Surfaces Volcanism I

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Page 1: PTYS 554 Evolution of Planetary Surfaces Volcanism I

PTYS 554

Evolution of Planetary Surfaces

Volcanism IVolcanism I

Page 2: PTYS 554 Evolution of Planetary Surfaces Volcanism I

PYTS 554 – Volcanism I 2

Volcanism I Mantle convection and partial melting Magma migration and chambers Dikes, sills, laccoliths etc… Powering a volcanic eruption

Volcanism II Magma rheology and volatile content Surface volcanic constructs Behavior of volcanic flows Columnar jointing

Volcanism III Interaction with volatiles (Maars, Tuyas etc…) Ash columns and falls, Surges and flows Igminbrites, tuffs, welding Pyroclastic deposits

Page 3: PTYS 554 Evolution of Planetary Surfaces Volcanism I

PYTS 554 – Volcanism I 3

Volcanoes on all the terrestrial bodies (and then some…)

Mercury – Smooth plains Moon – Maria Venus – Maat Mons

Earth – Mount Augustine Mars – Olympus MonsIo – just about everywhere

Page 4: PTYS 554 Evolution of Planetary Surfaces Volcanism I

PYTS 554 – Volcanism I 4

Volcanism on Earth Mostly related to plate tectonics Mostly unseen. ~30 km3 per year (~90%) never reaches the surface

Rift-zone and subduction-zone volcanism has very different causes

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PYTS 554 – Volcanism I 5

Volcanic material derived from the mantle Silicate composition built from SiO4 tetrahedra

Mantle rocks built from Olivine and Pyroxene

Olivine Isolated tetrahedra joined by cations (Mg, Fe) (Mg,Fe)2SiO4 (forsterite, fayalite)

Pyroxene Chains of tetrahedra sharing O atoms (Mg,Fe) SiO3 (orthopyroxenes)

(Ca, Mg, Fe) SiO3 (clinopyroxenes)

Page 6: PTYS 554 Evolution of Planetary Surfaces Volcanism I

PYTS 554 – Volcanism I 6

Partial melting Rocks (incl. mantle rocks) are messy mixtures of many minerals In a pyroxene-olivine mixture the pyroxene melts more readily than the olivine More silica-rich minerals melt even more readily Melting mantle at the Eutectic has a specific composition – generally basaltic

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Magma is characterized by silica and alkali metal content

Partial melting of fertile mantle produces basalts

Higher temperatures mean more Olivine is melted (lowers Si/O ratio)

Proportionally lower Silica in melt Proportionally more Iron etc…

Io volcanism probably ultramafic High-temp melting of Earth’s mantle in early

history produced Komatiite – primitive basalt

UltrabasicPrimative

Basic AcidicEvolved

Fe poorLight

Less-dense

Fe richDark

Dense

Page 8: PTYS 554 Evolution of Planetary Surfaces Volcanism I

PYTS 554 – Volcanism I 8

Recall that for the geotherm rolls over when radiogenic isotopes are in the crust

Steady-state solution: T = T0 + (Q/k) z – (H/2k) z2

When dT/dz=0 then z = Q/H ~ 100 km H~0.75 μW m-3 Q~0.08 W m-2

Ordinarily mantle material would never melt

Three ways to get around this (ranked by importance) Lower the pressure by moving mantle material upwards Change the solidus location (adding water)

Important only on Earth

Raise the temperature (plumes melting the base of the crust)

Page 9: PTYS 554 Evolution of Planetary Surfaces Volcanism I

PYTS 554 – Volcanism I 9

Mantle temperatures follow an adiabat α : Thermal expansion coefficient Cp : Heat capacity

Works out to only ~ 0.25-0.5 K/km Material rises and cools at this rate (i.e. not much) …but pressure drop is large

Material can cross the melting curve

Convection creates near isothermal mantle Temperature changes accommodated across

boundary layers Heat transport across boundary layer is

conductive Rates of cm/year

z

T

Lithosphere

ΔTh

δ<<h

Ignore the lithosphere/asthenosphere

boundary in this figure

Decompression melting

Page 10: PTYS 554 Evolution of Planetary Surfaces Volcanism I

PYTS 554 – Volcanism I 10

Most important mechanism for rift zones Requires a thin lithosphere Melting starts at ~50km

Mantle plumes can also create hot-spot volcanism with this mechanism Ocean island basalts

Accounts for ~75% of terrestrial volcanism …and probably 100% of planetary volcanism on other terrestrial planets

Page 11: PTYS 554 Evolution of Planetary Surfaces Volcanism I

PYTS 554 – Volcanism I 11

Adding water changes the melting point As solid stability increases

Olivine – isolated tetrahedra Pyroxenes – chains Amphiboles – double chains Feldspar – sheets Quartz – 3D frameworks

Water breaks the Si-O bonds SiO2 + H2O -> 2 Si OH Acts in the same way that raising temperature does

Descending slabs loose volatiles From hydrated minerals e.g. mica at 100km From decomposition of marine limestones Causes mantle melting – leads to island arc basalts

Melosh, 2011

Page 12: PTYS 554 Evolution of Planetary Surfaces Volcanism I

PYTS 554 – Volcanism I 12

Magma transport Mantle melt forms at crystal junctions

High surface energy

Wetting angle determines whether melt can form an interconnected network

<60° required for permeability

Less dense liquid flows upwards through the permeable mantle.

At mid-ocean ridges the asthenosphere comes all the way up to the base of the crust

Melt collects in magma chamber

Page 13: PTYS 554 Evolution of Planetary Surfaces Volcanism I

PYTS 554 – Volcanism I 13

Lithosphere

Things are harder when there’s a lithosphere No partial melting (otherwise it wouldn’t be rigid) so no permeable flow Pressures at the base of the lithosphere are too high to have open conduits

Magma ascends through the lithosphere (and oceanic crust) in dikes Fine as long as ρ(magma) < ρ(country rock)

Magma ascends to the level of neutral bouyancy

Tilling and Dvorak, 1993

Magma

Page 14: PTYS 554 Evolution of Planetary Surfaces Volcanism I

PYTS 554 – Volcanism I 14

What about under continents? Rising basaltic melt encounters continental

crust

Thin crust: basaltic volcanism still possible SW United states during Farallon subduction

Thick crust: Basalts don’t reach the surface Andes today Basalt underplates the crust and heats the continental

rock Melting produces felsic magma

Intermediate states are common so we have a wide variety of magma compositions in continental volcanism

Likewise for continental hotspot volcanism…

Under continental crust transport is harder Density change at the Moho Now ρ(magma) > ρ(country rock)

Magma chamber at the base of the crust

Felsic melts are still buoyant and can rise to form shallower magma chambers

Page 15: PTYS 554 Evolution of Planetary Surfaces Volcanism I

PYTS 554 – Volcanism I 15

Differentiation occurs within magma chambers

Minerals condense and fall to the floor Cumulates

Follows Bowens reaction series

Melts become more felsic

Volatiles no longer kept in solution H2O and CO2

Starts to build pressure in the chamber

Pressure can force out magma – Eruptions!

Intrusive eruptions cool slowly below the surface

Extrusive eruptions cool quickly on the surface

Discontinuous Continuous

Page 16: PTYS 554 Evolution of Planetary Surfaces Volcanism I

PYTS 554 – Volcanism I 16

Felsic magmas tend to have more water Water is a necessary component to form felsic melts and granites

Page 17: PTYS 554 Evolution of Planetary Surfaces Volcanism I

PYTS 554 – Volcanism I 17

Intrusive structures Sills Dikes

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Intrusive structures Laccolith – bows up preexisting layers, so shallow Lopolith – subsidence from overlying layers - deep

Page 19: PTYS 554 Evolution of Planetary Surfaces Volcanism I

PYTS 554 – Volcanism I 19

Batholith Many frozen magma chambers

Page 20: PTYS 554 Evolution of Planetary Surfaces Volcanism I

PYTS 554 – Volcanism I 20

Formation of bubbles Reduces magma density – helps magma rise to the surface Also increases viscosity

Less water in the melt - Allows silica to polymerize Expanding bubbles cool magma

Emptying the magma chamber causes decompression More volatiles degassed – faster ascent etc… Leads to a ‘detonation front’ that propagates downwards Runaway effect until the magma chamber empties

Magma shredded by exploding bubbles If volatile content is very high If viscosity is very high and bubbles can’t escape Generates volcanic pumice and ash

Page 21: PTYS 554 Evolution of Planetary Surfaces Volcanism I

PYTS 554 – Volcanism I 21

Volcanism I Mantle convection and partial melting Magma migration and chambers Dikes, sills, laccoliths etc… Powering a volcanic eruption

Volcanism II Magma rheology and volatile content Surface volcanic constructs Behavior of volcanic flows Columnar jointing

Volcanism III Interaction with volatiles (Maars, Tuyas etc…) Ash columns and falls, Surges and flows Igminbrites, tuffs, welding Pyroclastic deposits

Page 22: PTYS 554 Evolution of Planetary Surfaces Volcanism I

PYTS 554 – Volcanism I 22

Released volatiles power the eruption Injection of new magma Fractional crystallization Collapse of overburden Interaction with ground water Etc…